Ensure previous FLI frame is loaded before seeking to the next

Tests/test_file_fli.py

@@ -85,6 +85,13 @@ class TestFileFli(PillowTestCase):
         layer_number = im.tell()
         self.assertEqual(layer_number, 1)
 
+    def test_seek(self):
+        im = Image.open(animated_test_file)
+        im.seek(50)
+
+        expected = Image.open("Tests/images/a_fli.png")
+        self.assert_image_equal(im, expected)
+
 
 if __name__ == '__main__':
     unittest.main()

src/PIL/FliImagePlugin.py

@@ -131,6 +131,9 @@ class FliImageFile(ImageFile.ImageFile):
             self.__frame = -1
             self.__fp.seek(self.__rewind)
             self.__offset = 128
+        else:
+            # ensure that the previous frame was loaded
+            self.load()
 
         if frame != self.__frame + 1:
             raise ValueError("cannot seek to frame %d" % frame)